Abstract: As a new concept in ship design, ballast-free ship has been studied extensively abroad, but it is rarely mentioned in China. In this paper, the design and variation of the hull lines for this kind of ships were studi ed and a new design and variation method was proposed under the assumption that area of the mid-ship section equals to that of the mother ship. With this method, the hull lines of the bottom of the mother ship was chan ged into V-Shape, so that the designed ship has an inclined angle at the bottom with a wider breadth while the area of the mid-ship section was kept unchanged. The proposed new method is simple but reliable. It can obt ain smooth hull lines for the design ship with the major factors unchanged, such as length, draught and displacement, etc.